Sexual Size Dimorphism in Relation to Female Polygamy and Protandry in Butterflies: A Comparative Study of Swedish Pieridae and Satyridae

Abstract

In a number of insects male size has been shown to be positively correlated with mating success. Since variance in male mating success should increase with the degree of female polygamy, sexual selection for large male size may be stronger under polyandry than under monandry. To test this hypothesis we correlated male/female sexual size dimorphism, defined as male/female forewing length, with the degree of female polygamy, assessed by counting the number of spermatophores in wild-caught females. Both sets of data were assessed from 23 species of butterflies, 11 in the Pieridae and 12 in the Satyridae. There was a positive correlation between male/ female sexual size dimorphism and female polygamy in both families. Although size is affected by many factors, we contend that this result is best explained by sperm competition; in butterflies, the mass of the ejaculate delivered by a male is positively correlated with male size, and large ejaculates induce a longer period of female unreceptivity, resulting in sexual selection favouring large male size in species where females mate repeatedly. Although there is an obvious conflict between having a short development time and acquiring large size, the expected negative relationship between sexual size dimorphism and protandry was not statistically significant under direct development in the 8 species tested. This suggests that males under direct development achieve both large size and short development time simultaneously by increasing their growth rate. In diapause generations of 14 species (including overwintering generations of the 8 bivoltine species above) there was, however, such a negative relationship. Since protandry in diapause generations is mainly the result of morphogenesis of the adult during postdiapause pupal development, this is consistent with the tendency for small individuals, males, to require shorter morphogenetic development times than larger individuals, females. We found a weak negative correlation between polyandry and protandry, statistically significant only under diapause development. This suggests that mating with virgin females is more beneficial than mating with non-virgins also under female polygamy, and indicates that selection for early male emergence is only weakly relaxed in species where females mate repeatedly.